Science 338, 666–671 (2012)

Ribosome biogenesis occurs in the nucleolus and involves the ordered assembly of rRNA transcripts with ribosomal proteins synthesized in and imported from the cytoplasm. Kressler et al. now show that two ribosomal proteins, Rpl5 and Rpl11, which bind near each other on the 5S rRNA of the 60S ribosomal subunit, are brought into the nucleus together by a newly defined transport system. Using tandem affinity purification, the authors showed that a new protein, symportin 1 (Syo1), purifies together with Rpl5 and Rpl11. Syo1 contains ARM and HEAT repeat domains typically found in nuclear transport proteins and is conserved in eukaryotes. Knockout of yeast syo1 results in lowered 60S ribosomal subunit abundance, supporting its role in ribosome assembly. Yeast two-hybrid and in vitro reconstitution experiments revealed that Rpl5, Rpl11 and Syo1 form a heterotrimeric complex. Crystallography and hydrogen-deuterium exchange MS defined Syo1 interactions with Rpl5 and Rpl11, respectively. Though Syo1 readily transits the nuclear pore complex, the Syo1–Rpl5–Rpl11 complex requires Kap104, a nuclear transport receptor, for efficient nuclear import and Ran-GTP for release of the complex from Kap104. Released Syo–Rpl5–Rpl11 binds efficiently to 5S rRNA. Taken together, the data support a potentially general model in which coordinated transport of ribosomal proteins can be used to order the assembly of preribosomal complexes.